Optical codes, notably barcodes, are widely used at present to identify items, for example by being directly printed onto packaging or by being present on labels affixed to the items or their packaging. Optical codes are also found on various documents, notably security documents.
The term “security document” denotes a means of payment, such as a banknote, a check or a meal voucher, an identity document such as an identity card, a visa, a passport or a driver's license, a lottery ticket, a transport document or an admission ticket to a cultural or sporting event.
The article in the RFID Journal, “Cent RFID Tags for Supermarkets” by Bob Violino, states that there have been proposals to print barcodes on a paper in which conductive fibers have been dispersed, thereby permitting both an optical reading of the barcode and the creation of a unique identifier by means of the electromagnetic signature of the paper.
The publication WO 2012/005733 A1 discloses a 2D barcode formed by using a conductive ink. A chip including a short-range antenna is electromagnetically coupled to a longer-range antenna formed with the 2D barcode.
The application US 2005/0284941 A1 describes the forming of a barcode from the same material as at least a portion of an RFID communication device, for example from a sheet of conductive material that has been subjected to etching. The publication US 2007/0057054 A1 contains similar teachings.
The application EP 1 065 623 A2 discloses an electromagnetically readable barcode intended to replace optical barcodes and to overcome the difficulties encountered with the latter, such as dirt or obstructions that impede optical reading.
The application FR 2 956 232 concerns a chipless passive RFID label, comprising a plurality of separate parallel conductive strips formed on an electrical backing, with conductive links interconnecting neighboring conductive strips in such a way that the set of resonance frequencies of the label forms an identification code.
The publication WO 03/032242 A1 teaches the forming of a barcode with an ink containing light-emitting and/or electrically conductive substances. Printing is carried out with different thicknesses, enabling signals of different intensity to be generated.
The article entitled “Evaluation of conductive inks for anti-counterfeiting deterrents” by Jason S. Aronoff and Steven J. Simske mentions that the use of conductive ink may increase the number of ways of authenticating information printed on a package or a label. The author examines the print quality of a magnetic ink and of a novel silver-based conductive ink. The use of a 2D Data Matrix barcode is proposed as a test means for verifying the print quality. The article studies the effect of pre-compensation, a method in which the size of the print area of each elementary black square of the Data Matrix is reduced, as well as the effect of the substrate, on the barcode authentication success rate.
The article “Printed Antennas for Combined RFID and 2D Barcodes” by Steven J. Simske, Jason S. Aronoff, and Bobby Duncan discloses the use of a portion of a 2D barcode as an RFID antenna. Various possibilities for the forming of the antenna are mentioned, notably:                the fact that two distinct inks having the same color are used, one ink being conductive, for forming the antenna, while the other is non-conductive, for forming the portion of the code bearing the data,        the use of the same ink or the same conductive ink precursor to print the whole of the 2D barcode, only the portions to be used as an antenna being activated,        the printing of the 2D barcode entirely in a conductive ink and the forming of fine breaks to isolate the portion forming the antenna from the portion bearing the data.        
The article entitled “RFID Fibers for Secure Applications” authored by Jonathan Collins discloses the use of resonant nanofibers integrated into a paper backing to create a unique authentication key.
The article by E. Perret, M. Hamdi, A. Vena, F. Garet, M. Bernier, L. Duvillaret, and S. Tedjini, “RE and THz Identification using a new generation of chipless RFID tags,” Radioengineering—Special Issue towards EuCAP 2012: Emerging Materials, Methods, and Technologies in Antenna & Propagation, Vol. 20, N° 2, pp. 380, 386, June 2011, describes a chipless RFID label including a combination of C-shaped coplanar strips and short-circuits, the set of resonance frequencies of the label forming an identification code. To broaden the frequency range available for detecting encoded information, the article also discloses a new type of chipless label having a multi-layered structure allowing operation in the THz range.
The document EP 1 675 040 A1 describes a marking having a plurality of areas, the marking having magnetic, electrical and/or electromagnetic properties differing from one area to another.
FR 2 899 361 A1 discloses a method of associating an authentication information element of a substrate with a barcode carried on the substrate.
WO 03/019502 A1 discloses a label having an optical code and a magnetic code, these codes being complementary so as to reinforce the security of the label.
WO 2006/108913 A1 discloses an electromagnetically readable marking, including areas having different electrical conductivities, in order to provide a large amount of information contained in the marking.